1. Field of the Invention
The invention relates to radio frequency identification (RFID) technology, and in particular, to communications with RFID tags.
2. Background Art
Radio frequency identification (RFID) tags are electronic devices that may be affixed to items whose presence is to be detected and/or monitored. The presence of an RFID tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored wirelessly by devices known as “readers.” Readers typically have one or more antennas transmitting radio frequency signals to which tags respond. Because the reader “interrogates” RFID tags, and receives signals back from the tags in response to the interrogation, the reader is sometimes termed as “reader interrogator” or simply “interrogator” or “reader.”
With the maturation of RFID technology, efficient communication between tags and readers has become a key enabler in supply chain management, especially in manufacturing, shipping, and retail industries, as well as in building security installations, healthcare facilities, libraries, airports, warehouses, etc.
One of the most significant concerns of RFID system design is the optimization of tag throughput rates. The number of tags successfully processed per second has a direct impact on the feasibility of RFID in many applications. When interrogating a large population of tags, some of the most important parameters are the bit data rate of the tag-to-reader channel, the ability of the protocol to minimize collisions, and the amount of data to be transferred from each tag. For a given bit rate and protocol, an implementation that minimizes the amount of over-the-air data transfer will have a distinct competitive advantage. Some of the data transfer is “overhead” (polling, acknowledging, select commands, etc), but a large percentage is a tag's “payload,” such as the serialized EPC number in retail tags. Of that payload, a large and growing percentage is devoted to the serialization portion which is unique down to each actual item. Item-level uniqueness is one of RFID's major advantages over bar coding, and many new RFID applications will undoubtedly make good use of this capability. Being able to track item-level uniqueness also raises both security and privacy issues. From an implementation standpoint, encryption resembles compression but without a decrease in size.
However, many instances of current inventory practice tend to ignore serial numbers, and track only down to Stock Keeping Units (SKUs) or the equivalent. For this and many other current and future RFID applications, the serial number portion of each tag's identifier (sometimes called ID) is “thrown away,” but the communication of this unused data from every tag within range of the reader still uses up a significant portion of the air interface bandwidth.
For example, in current practice when 96-bit EPC Generation 2 (Gen 2) data specification tags are used for identifying individual cases on a pallet, each tag encodes a “sGTIN-96” identifier. For that identifier, almost 40% of the payload bits are devoted to the serial-number portion. The serial number portion is not needed in many inventory applications, and is discarded. This inefficiency may significantly worsen in future practice. In the near future, tags will use the full-capacity “sGTIN-198” version of the identifier. In this case, nearly 71% of the payload is devoted to serialization.
In other applications, the serial number information is needed and thus is not discarded. However, the number of transmitted bits of serialization data defined in the Gen 2 protocol was optimized for simplicity, not speed. For example, the alphanumeric data in an sGTIN-198 identifier is represented and transmitted at seven bits per character. More complex but more bit-efficient encoding schemes are known in the art, such as the “ISO 646 Encodation Mode” of the EAN.UCC Composite symbology. This mode supports the full character set in the serial number, but it uses only needs four bits per decimal digit, and seven bits per alphabetic character. More bits are needed only for the rarely-used punctuation characters.
A need for reducing the transmitted payload is present. In the current EPC Gen 2 case, once a reader has transmitted a selection mask, so that, for example, only tags whose EPC begins with “11010” are allowed to reply, then the transmitted tag replies do not need to include the initial “11010” because the reader already knows that all valid replies will begin with the selected bit pattern. Thus, the EPC Gen 2 spec provides an explicit reader command to the tags to truncate their replies by leaving off the known leading portion of their identifier, thus reducing transmission times. The truncated reply still includes the CRC-16 as calculated over the entire ID, and the reader must prepend the known leading bits to the actually-transmitted bits in order to validate the transmission.
Thus there exists a need to reduce the amount of bits transmitted by tags during RFID communications while still maintaining compatibility with RFID communications standards.